1. Technical Field
The present invention relates to an image synthesizing apparatus, an image synthesizing method and an information storage medium.
2. Description of the Related Art
There are known many types of image synthesizing systems in which an object to be displayed is formed by polygons(primitive surfaces), such objects being used to synthesize an image. For example, in a three-dimensional racing game system, polygons are used to form other racing cars, guard-rails, signboards, roads, etc. to be displayed on a screen. A player operates its racing car using given operating means (steering wheel, accelerator pedal, game controller and others) for enjoying a car race with the other racing cars. In such an image synthesizing system, the viewpoint and sight line of the player vary depending on the operation of the player. This causes such a situation that the back side of the polygons will be displayed on the screen. Assuming a case that a signboard formed by polygons is to be displayed on the screen, the front side of the polygons is only displayed on the screen when a racing car on the screen goes forward. If the racing car on the screen goes backward, the back side of the polygons will be displayed.
In an image synthesizing system of such a type that the back side of the polygons will not be displayed on the screen, for example, a signboard will vanish from the player's field of view instantly when the player's racing car is turned to the reverse direction. This is very unnatural for the player.
On the other hand, another image synthesizing system of such a type that faces of a pair of polygons each used to form the back and front side of a signboard to be displayed are put together, raises another problem in that the number of polygons used to form an object is duplicated. Such an image synthesizing system is required to provide a real-time operation in which an image is synthesized in real time for the player's operation. When the number of polygons to be processed is increased, the processing load is correspondingly increased. Thus, the real-time operation will not be maintained, resulting in deterioration of the image quality.